plugins: fix uninitialized $MUNIN::VERSION

Commit 733f01b1ad48c1e59f0804318a90615429f68e77 introduced a
specific user agent for munin plugins.

But it forgot to import the Munin::Plugin package.

Closes: D:717265
Merge branch 'fix/d/717265-use-uninit-val-const' into stable-2.0

Conflicts:
	plugins/node.d/nginx_request.in
	plugins/node.d/nginx_status.in
2.0.17
Substitute some @@ vars in generate files. Debian: #665688

This is a hacky solution to this problem as it uses a list of files to
substitue the variables in instead of fixing the root cause.
Merge pull request #122 from mattthias/stable-2.0

Substitute some @@ vars in generated files. Fixes Debian Bug #665688
Merge branch 'missing-configuration-keywords' into stable-2.0
plugins/open_files: fix overrided used.warn/crit

As reported by Helmut on IRC:

    < helmut> open_files from munin mainline contains both a "print_warning used"
              and a manual awk .. print used.warning
    < helmut> that can result in used.warning to be printed twice with different
              values